One commonly used method of steering an articulated machine utilizes a pair of hydraulic cylinders that are connected between the front and rear frames of the machine on opposing sides of the articulation point. The cylinders can be selectively actuated to extend one cylinder while retracting the other to pivot the front frame of the machine with respect to the rear frame and thereby provide steering. Pressurized fluid is normally directed to the cylinders by a pump known as a hand metering unit (HMU) that is positioned at the base of the steering column. With the revolution of the steering wheel in either direction, fluid is pumped from the HMU to the steering cylinders to obtain the desired steering function. The more revolutions imparted to the steering wheel, the greater the steering correction. This system has been known to work quite well in several machine applications for many years.
It has been a recent trend in the construction industry to focus on the needs and the comfort of the operator of construction machines. This focus not only addresses the ergonomics of the operator's surroundings but also those functions of the operator that will cause fatigue. In some machines, such as wheel loaders, the method of steering the machine is one known source of operator fatigue. In one mode of normal operation, such as truck loading, the operator must move the machine between a pile of material and a truck into which the material is deposited for subsequent removal from the area. This maneuvering is usually done in a very compact area to reduce the time it takes the machine to cycle between the material and the truck. During this operation, the operator is constantly rotating the steering wheel to achieve maximum machine articulation. One solution to this problem has been to employ various electronic methods to control the flow of pressurized fluid to the steering cylinders. This eliminates the need for the operator to rotate the steering wheel to achieve a pumping action. In some instances, a steering actuator is provided that may or may not be a steering wheel that incorporates the electronic controls. In this type of steering actuator, small degrees of movement will typically result in maximum degrees of machine articulation. This greatly reduces the amount of motion required by the operator and thereby the reduces the resulting fatigue. While this method is highly successful, it does not necessarily lend itself as well to other modes of operation where the machine must travel over a roadway for substantial distances to move the material. This operation is generally known as "load and carry". In this mode it is desirable to provide the operator with a more "automotive" feel to the steering actuator to allow larger amounts of movement to provide smaller amounts of correction for the machine as it moves at higher speeds. Since the electronically controlled steering system is set up to provide quick response to reduce cycle time, the ride under roading conditions may experience rather jerky corrections making the ride rather uncomfortable and contributing to operator fatigue of a different variety.
Another disadvantage that exists with a steering system that utilizes a HMU is the requirement for the transmission controls to be separated from the steering controls. Since the steering wheel requires several rotations to achieve larger turning angles, the transmission control must be located in a location in close proximity to the steering wheel so they may be manipulated by one hand while the operator steers with the other. This only adds to the "busyness" of the operator which again adds to his or her fatigue.
The present invention is directed to overcoming one or more of the problems as set forth above.